Optical transmission is widely used as a communication means because of its characteristic feature that it has a large capacity and it is entirely insusceptible to the influenced of electromagnetic noises. Optical fibers which are now being practically use as light transmission media can be classified into quartz optical fibers and plastic optical fibers (hereinafter referred to as "POFs").
POFs have the following advantages: (1) They are flexible and easily handleable in spite of their large diameter. (2) When coupled to other POFs and light-emitting devices, they have such great latitude in positioning that they can be easily connected without expensive equipment. (3) No optical system is required for connecting purposes.
Owing to these advantages, the application of POFs to short-distance communication means (e.g., data links), sensors and the like is spreading. Moreover, it is expected for the future that they will be applied to low-cost short-distance information transmission lines having many junction points, including terminal wirings in station networks and subscriber networks (FTTHs) such as LANs among FA and OA equipment located on the inside and outside of the floor. Furthermore, POFs have such excellent flexibility that they are scarcely subject to failure, breakage or deterioration even in a vibratory Ienvironment and are also superior to quartz optical fibers in this respect. Accordingly, attempts are being made to apply POFs to signal communication lines including, for example, networks in vehicles such as automobiles, electric cars and airplanes.
Meanwhile, there is a yearly increasing demand for an improvement in transmission speed in these communication applications. When bandwidth is used as an index indicating the quantity of data which can be transmitted through a POF in a unit time, the bandwidths of POFs heretofore in practical use have usually been at most 5-6 MHz.multidot.km (-3 dB). At this bandwidth, the practical communication rate at a fiber length of 100 m is 100 Mbps (bits per second) or less, even if an optimized LED module is used. Thus, this bandwidth is too narrow for applications such as medium-speed and high-speed LANs.
There have been proposed various SI type POFs using polymethyl methacrylate (PMMA) as the core material and a fluorine-containing polymer as the cladding material.
Japanese Patent Laid-Open No. 36111/'84 discloses a cladding material comprising a copolymer composed of a long-chain fluoroalkyl methacrylate, methacrylic acid and methyl methacrylate. However, this cladding material contains not less than 65% of the long-chain fluoroalkyl methacrylate, and the content of methyl methacrylate is not greater than 29%. Consequently, the refractive index of the cladding material is not greater than 1.420 and the numerical aperture of the resulting optical fiber is not less than 0.45. This reference includes no suggestion on a POF having wide bandwidth characteristics.
Moreover, Japanese Patent Laid-Open No. 66706/'86 discloses a cladding material comprising a copolymer composed of a long-chain fluoroalkyl methacrylate, a short-chain fluoroalkyl methacrylate and methyl methacrylate. However, the content of methyl methacrylate in this cladding material is not greater than 50%, and this reference includes no suggestion on a POF having wide bandwidth characteristics.
Recently, it has been proposed in Conference Proceedings of 3rd International Conference on Plastic Optical Fibers & Applications, pages 148-151 and 147 that wide bandwidth characteristics can be imparted to an SI type POF by reducing the numerical aperture of the fiber.
Furthermore, Japanese Patent Laid-Open No. 239420/'95, which was laid open to the public after the filing of Japanese Patent Application No. 116174/'95 underlying the present application for the declaration of priority, discloses a POF using PMMA as the core material and a copolymer composed of a specific fluoroalkyl methacrylate and MMA as the cladding material. This reference shows the facts that a wide bandwidth can be achieved by making the refractive indices of the core material and the cladding material close to each other and that the thermal resistance of the POF can be improved by using a specific fluoromethacrylate copolymer as the cladding material. However, this POF contains a short-chain fluoromethacrylate in the cladding material and hence has unsatisfactory mechanical properties.
On the other hand, POFs having a three-layer structure comprising a core, a cladding and a protective layer have conventionally been proposed. Japanese Patent Laid-Open No. 204209/'87 discloses a POF in which two layers of a cladding material consisting of a fluorine-containing resin are disposed on a core material consisting essentially of PMMA. In this patent, it is intended to improve the thermal resistance of the POF by using two cladding layers and, moreover, improve the flexing resistance of the POF by reducing the thickness of the first cladding layer to as small as 3-4 .mu.m. However, this POF also contains a short-chain fluoromethacrylate (such as pentafluoropropyl methacrylate or tetrafluoropropyl methacrylate) in the cladding material and hence has unsatisfactory mechanical properties. Moreover, this reference disclosed no design concept concerning an SI type POF having wide bandwidth characteristics.
Japanese Patent Laid-Open No. 249325/'93 discloses a POF comprising a core material consisting essentially of PMMA and a cladding material consisting of a specific fluororesin. This reference proposes the idea that two cladding layers may be used to prevent the plasticizer contained in the jacket material from diffusing into the core and the cladding and also to reduce the transmission loss of the POF and to prevent the deterioration of mechanical properties. However, the refractive index of this cladding material is not greater than 1.409 and, therefore, the numerical aperture of the resulting optical fiber is not less than 0.48. Thus, this reference also includes no suggestion on a POF having wide bandwidth characteristics.
As described above, a POF having a reduced numerical aperture serves as a means for obtaining an SI type POF having wide bandwidth characteristics. However, a reduction in the numerical aperture of a POF raises the following problems: (1) The amount of light released from the side surface of the fiber to the outside is increased when the fiber is bend, resulting in an increase in light transmission loss; and (2) the coupling loss between the POF and the light source is increased. Moreover, it is also important to secure practical mechanical strength of the POF.